In inductively coupled mass spectrometry (ICP-MS), a sample is fed into a plasma that is maintained in an excited or energized state by inductive coupling. Typically, the plasma gas is argon. The plasma typically comprises the analyte, usually a metal and usually ionized, and various other constituents, such as argon, oxygen, hydrogen and also water vapor, all of which will commonly be neutral but some (about 0.1%) may be ionized. For wet plasma, which is typically used, the content of the reactive neutrals such as H, O, and their various polyatomic combinations, is as high as 17%. The plasma, including these ions and neutrals, passes into a chamber maintained at approximately 4 Torr. From this chamber, the plasma passes through a skimmer into a chamber maintained at a low pressure off approximately 10−3 Torr. From this chamber, the ions are intended to pass into a reaction/collision cell. The reaction/collision cell commonly has a multipole rod set, and can be maintained at different pressures; for example when no reaction is required, it may be maintained at 10−5 Torr, while a pressure of 5×10−3 Torr to 10−2 Torr is provided by a reaction/collision gas when reaction or collision induced dissociation (CAD) is required. The higher pressure is maintained in the reaction cell when it is desired to promote ion-molecule reactions or CAD. In such a case, a simple analysis would suggest that the higher pressure within the reaction cell would prevent neutral species from passing into the reaction cell, and only ions, driven by the potential gradient through the whole instrument, would overcome the pressure difference and pass into the reaction cell. However, this overlooks the significant velocity created by the expansion of the plasma from the atmospheric pressure to a region at 4 Torr, which creates a suspension expansion jet. Consequently, individual ions and neutrals within the supersonic expansion jet, after passing through a skimmer into the region at 10−3 Torr, may have sufficient kinetic energy to overcome the pressure differential between the higher pressure in the reaction/collision cell and lower pressure of the region at 10−3 Torr, and pass into the reaction/collision cell. More specifically, and as detailed below, the present inventors have now realized that it is possible for neutral species to pass into the reaction/collision cell.
Ion-molecule reaction cells are widely used in ICP MS. Their successful operation depends on how pure the reaction gas is. Inductively coupled plasma is the source of neutral particles, because 99.9% of the gases that constitute the plasma are not ionized. Usually, about 4×1018-2×1019 molecules/s−1 flow of neutral plasma particles enters the mass spectrometer, which is equivalent of 0.1-0.4 scc/s. If these neutral gas particles are entrained into the flow into the reaction cell, the reactions are not controlled anymore. Instead of the high purity reaction gas introduced on purpose to the cell, it now has a mixture of the reaction gas with entrained plasma gases, and these plasma gases constitute up to 17% of the reactive neutrals H, O and various polyatomic combinations of these. Despite the fact that the pressure in the pressurized cell (which typical flow of 0.03-0.3 scc/s) may be higher than the background pressure of the vacuum compartment where the cell is positioned, the gases from the plasma can still enter the cell, because, as noted, the plasma gas undergoes supersonic expansion in the plasma-vacuum interface, after which particles travel with the terminal speed of about 2,300 m/s, typically. The impact pressure of such high velocity gas particles can be sufficiently higher than the pressure of the reaction gas in the cell, so the neutral gas particles from plasma will be entrained into the reaction cell.
Similar processes are taking place in any other mass spectrometers, in which the ion source pressure is sufficiently higher than the pressure in a collision/reaction cell. A variety of the instruments now comprise collision devices for collisional cooling, collisional focusing or collision-induced dissociation. For example, in Electrospray Ionization Mass Spectrometry, the ion source is usually operated at atmospheric pressure, from which ionized and neutral particles are delivered into the lower pressure collision cell by a supersonic expansion. As noted above, the impact pressure of the expanding ion source gas may be greater than the collision cell pressure, so that the neutral gas particles from the ion source will be entrained into the collision cell, altering the composition of the collision gas. As a result, un-predicted and un-controlled dissociative and reactive collisions with the collision gas of altered composition may bring undesirable modifications to the ions that are to be detected by mass analysis.
A variety of ion-molecule reactions in pressurized mass-analyzing and ion transmitting devices have been successfully used in ICP Mass Spectrometry for chemical resolution of analyte ions from isobaric interfering species by use of a reaction cell. Douglas [Douglas, D. J. Canad J. Spectrosc. 1989, 34, 38] was first to report on discrimination between the rare earth elements and their oxides through the specificity of oxidation by the reactive gas. Tb+ was shown to oxidize more readily with O2 than CeO+. The analyte ion (159Tb+) was moved to a higher m/z and could thus be measured as TbO+. The interfering ion (142Ce17O+) was not shifted to the same extent, thus providing a possible analytical advantage of achieving better signal-to-noise ratio for Tb signal measured as TbO in the presence of Ce in the sample. Shortly after, Rowan and Houk [Rowan, J. T.; Houk, R. S. Applied Spectrosc. 1989, 46, 976] reported on the removal of the interfering argide ions from the m/z of analyte ions of interest due to lower reactivity of the latter towards reaction gas such as CH4.
The specificity of the analyte-interference chemical resolution in general and in both of the above-described cases is dependent on the reaction gas properties. When the interfering species are to be moved away from the m/z of the analyte ion, the reaction gas reactivity towards the analyte is desirably low, while being high towards the interfering species. On the other hand, when the analyte ion is to be moved from its m/z by conversion to a polyatomic ion, the reactivity of the gas towards the analyte ion should preferably be high and simultaneously should be low towards the interfering species. In the latter case, the reaction that converts the analyte ions should preferably have one or only few channels, so that the analyte ion current or signal is not distributed amongst many product ion currents and the detection capabilities are not compromised. The reactivity of the gas towards the interference should in this case be low, at least for any reaction channels that can produce from the interference product ions at the same m/z as that of the analyte product ions, i.e. one does not want any interference products to be isobaric with analyte product ions.
The inventors have recently shown that the highest effectiveness of reactive isobaric interference removal in ICP MS can be achieved only if the average number of ion-molecule collisions in the pressurized device is sufficiently high. Efficiency of 109 of suppression of Ar+ signal by reaction with NH3 has been demonstrated with an average number of collisions of >20. This high efficiency of reactive removal of the interferences was shown to be accompanied by promotion of sequential reaction chemistry that produces multiple new species in the cell.
The present inventors have also realized that this sequential chemistry can be controlled and used, to eliminate undesired interferences. This is implemented by a technique, designated by the assignee, as a Dynamic Reaction Cell. Briefly, this requires the provision of voltages to the quadrupole rod set of the reaction cell, to provide a band pass, thereby ejecting ions outside the set pass band. This technique is described in more detail in WO98/56030, to the assignee of the present invention.
Persons skilled in this art will understand that the purity of the reaction gas, supplied to the reaction cell, is crucial for efficient control of reaction chemistries in the pressurized reactor. Research grade high purity (99.999%) gases are preferable. Yet, as indicated above, the present inventors have realized that the biggest possible source of contamination of the reaction gas resides in the mass spectrometry system itself. The plasma-vacuum interface necessarily causes large amounts of neutral molecular and atomic gases from the ion source (Ar, O, O2, H, H2, H2O) to enter the vacuum chamber. It is a well known fact that the degree of ionization of the plasma sustaining gases in ICP is low (0.04-0.1%), and thus the majority of the plasma species are neutral. Such partially ionized plasma-gas mixture enters the chamber at a high velocity, which is related to the terminal velocity of the supersonic expansion jet formed behind the skimmer interface. This velocity determines both neutral and ionized components trajectories, at least during the initial stages of the partially ionized gas propagation in the vacuum system. It may thus be said that the ionized and neutral components are coupled (their trajectories are co-defined by the same factors). The high velocity neutral gas particles may penetrate into the reaction chamber if it is positioned in line with their trajectories.
To applicants and assignee's knowledge, many other users of ICP MS with a reaction cell intend the reaction cell to remove unwanted interferences, without affecting the analyte. Commonly, the analyte is a metal, which is intended to be detected directly, i.e. without previous reaction to some compound thereof. As such, the issue of contaminants in the reaction gas reacting with the metal is a concern, as common analytes may react readily with major contaminants; for example many metals react significantly with water to form an oxides, thus compromising detection capabilities of the metals.
On the other hand, the assignee of the present invention has recently started to promote the use of oxides for detection. For this purpose, N2O, or other suitable reaction gas is provided in the reaction cell, to promote the conversion of analyte metal ions to their oxides. As noted above, for Tb as example, this can give improved results and eliminate problems due to isobaric interferences. However, a potential disadvantage with this technique is that oxides may react more readily with contaminants introduced from the plasma gas flow. For example, water vapour may convert an oxide to a hydroxide.
For example Rb and Sr have similar isotopes at m/z 87. Their ratio is widely used for measuring the age of the rock samples in geochronoly. To distinguish between them in ICP MS, Sr+ is oxidized by reactions with N2O, to give 87SrO+ at m/z=103. N2O is non-reactive towards Rb+, so that 87Rb+ does not oxidize readily and stays at m/z=87. Sr also has other isotopes at m/z=86 and 88. SrO+ reacts with water to form 86 SrOH+ at m/z=103. If any of water is entrained in the reaction gas by the processes described above, the detection of 87Sr+ as 87SrO+ is compromised by the interference from 86 SrOH+.
It is thus the purpose of the present invention to provide apparatus and method for controlled ion-molecule reactions in ICP Mass Spectrometry, that would ensure that predictability and specificity of the desired reaction chemistry in ion-molecule reactor is not compromised by uncontrolled dilution of the reaction gas by gas particles and other neutral species originating from the plasma or plasma-vacuum interface. Although described predominantly for use with an ion-molecule reactor and ICP plasma, the invention is not limited to this particular configuration and may be used in any device where neutral species can enter pressurized CAD or reaction chamber and promote reactions or collisions of ions with undesirable neutral species.
There are ICP MS devices on the market that have the reaction/collision cell in the direct sight of the neutral particles that propagate from the plasma (Micromass Platform and VG ExCell). The promotion of oxidation reactions on the VG Excell collision cell pressurized with He or He—H2 mixture was shown in presentation by J. Godfrey, I. B. Brenner, P. Sigsworth and J. Bathey [Paper F7, 2000 Winter Conference on Plasma Spectrochemistry, Fort Lauderdale, Fla., Jan 10-15, 2000], which indicates that the collision gas also contained other than He and H2 species, most likely entrained from the plasma gases.
There are various known proposals, either in patents or in commercially available devices, that improve the stability and reduce background count rate of the conventional ICP MS by removing the plasma particulates and photons from the direct sight of ion optics and/or detector. These include: photon stops and shadow stops (U.S. Pat. No. 4,746,794), Omega lens (Agilent HP7500 Series ICP MS, as shown in Agilent Technologies Inc. Publication # 5968-8813E, December 1999) or chicane lens (VG Excell, as was described by Jonathan Batey of VG Elemental in the presentation # 55 “Incorporating Collision Cell Technology into a Quadrupole ICP MS” at the 26th Annual Conference of the Federation of Analytical Chemistry and Spectroscopy Societies, Vancouver, Oct. 25, 1999), 90-degrees sector ion deflector (Hitachi ICP-ITMS, as described by Takayuki Nabeshima et al of Hitachi Ltd in the presentation FP34 “Development of Ion Trap Mass Spectrometer with Plasma Ion Source” at the 2000 Winter Conference on Plasma Spectrochemistry, Fort Lauderdale, Fla., Jan. 10-15, 2000) and off-axis transfer optical system (SPQ 9000 of Seiko Instruments, as shown in “Inductively Coupled Plasma Mass Spectrometry”, ed. A. Montaser, Wiley-VCH 1998, p.428). All of those are used to either stop the photons and neutral plasma particles from reaching the detector and/or ion optical elements in order to improve stability and background.
Most importantly, none of these known proposals are used to prevent the plasma neutral particles from entering the reaction/collision cell. One exception is ICP MS Dynamic Reaction Cell (DRC) by the assignee of the present invention. This instrument uses a “shadow stop” to stop the neutral plasma particles from contaminating the ion optical elements (as disclosed in U.S. Pat. No. 4,746,794 assigned to MDS), and also serves as a photon stop. However, its effect on neutral plasma gases was not appreciated. For reasons given above, it was previously believed that it was only necessary to prevent photons from reaching the detector, and large metal particles, that originate from incompletely disintegrated sample, from contaminating downstream ion optics components. In a commercial ICP-MS, penetration of the neutral gas particles into the ion optics poses no significant difficulty. Further, it was not realized that neutral gas particles, including the plasma gas, could be a significant problem, as these particles are not charged and there should be no potential driving them further into the mass spectrometer. This analysis overlooks the effect of the supersonic expansion jet which is now realized to be important. Thus, it is now appreciated that this stop also serves the purpose of stopping the plasma gases from being entrained into the cell.
This effect has not been appreciated before. Indeed, it has recently become apparent that instruments made by the assignee do not promote unwanted formation of oxides to the same extent as instruments from other manufacturers. However, the reason for this was not recognized. It is now believed that this “shadow stop” prevents the plasma gas entering the collision cell. In contrast, in instruments from other manufacturers, it is believed that contamination of the reaction gas with the plasma gas, promotes reaction of oxides, as their “stopping” devices are positioned behind (as opposed to being in front of) the reaction cell, which for them is thus in a direct line of sight of the high velocity plasma neutral particles.